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K-Ar ages of some volcanic rocks from the Cook and

G. BRENT DALRYMPLE U.S. Geological Survey, Menlo Park, California 94025 R. D. JARRARD* Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, California 92037 D. A. CLAGUE* Geological Research Division, Scripps Institution of Oceanography, La Jolla, California 92037

ABSTRACT northwest and tend to be youngest at the ing of volcanoes in the chain seemed neces- east-southeast end. He suggested that these sary, both to confirm the reliability of pre- K-Ar age measurements on 19 volcanic chains were formed by relative motion be- vious K-Ar dating and to provide a stronger rocks from , Mangaia, , tween the lithosphere and a hot spot (or test of the general melting-spot hypothesis. and in the Cook-Austral chain do melting spot) in the mantle. Morgan (1972) This paper presents the results of some ad- not show a systematic increase in the age of offered the hypothesis that these hot spots ditional K-Ar measurements and their im- the volcanoes to the west-northwest away were fixed, both relative to one another and plications for the origin of the Cook- from Macdonald as predicted by to the Earth's spin axis, and that they were Austral volcanic chain. the melting-spot hypothesis and suggested caused by mantle convection in the form of Age measurements were by the conven- by geomorphic evidence. Ages determined narrow plumes that brought material up- tional K-Ar method with the use of previ- for alkalic samples from Rurutu ward from near the core-mantle boundary. ously described techniques (Dalrymple and range from 1.02 to 1.09 m.y., for Mangaia Other mechanisms proposed for the origin Lanphere, 1969; Ingamells, 1970). Ar from 16.6 to 18.9 m.y., and for Aitutaki of linear island chains include propagating analyses were by isotope-dilution mass from 0.66 to 0.77 m.y. Two distinct periods fractures (Betz and Hess, 1942; Green, spectrometry; K was measured using of volcanism on Rarotonga were dated at 1971; Jackson and Wright, 1970; Vogt, lithium metaborate fusion and flame 1.8 and 1.2 m.y. B.P. The relation between 1974), diapiric upwelling (McDougall, photometry. the dated units and the main shield-building 1971), and melting caused by shear beneath stage of these volcanoes is uncertain and the lithosphere (Shaw, 1973; Shaw and PETROGRAPHY AND will remain so until better data on the erup- Jackson, 1973). Morgan (1972) suggested SAMPLE LOCATIONS tion history and mode of formation of vol- that the Austral-Southern Cook-Gilbert- canoes in the chain are available. Key Marshall band of islands and re- Thin sections of 46 samples from Rurutu, words: geochronology, potassium-argon, sulted from motion of the Pacific plate over Mangaia, Rarotonga, and Aitutaki were , Austral Islands, volcanic a melting spot now located at Macdonald examined to determine their suitability for chain, melting spot, Pacific plate, petrog- Seamount. K-Ar dating according to criteria discussed raphy, volcanology, basalt. Regardless of the exact dynamic by Mankinen and Dalrymple (1972). For mechanism proposed for melting spots, a Rarotonga, from which numerous speci- INTRODUCTION corollary of the general kinematic mens were available, samples were chosen hypothesis is that the ages of islands and to represent a broad range of rock types The Austral and Southern Cook Islands seamounts in each chain generally increase and the several stratigraphic units exposed are part of a quasi-linear volcanic chain of away from the present location of the melt- on the island (Wood and Hay, 1970). A seamounts and islands in the South Pacific ing spot. The ages of individual volcanoes table1 describing the general petrographic Ocean (Fig. 1); the chain extends west- within a chain thus provide an important features of the dated samples is available northwest for more than 2,000 km from test of the general melting-spot hypothesis. from The Geological Society of America Macdonald Seamount, which is thought to K-Ar dating of volcanoes in the Hawaiian- depository. Rock names used in Table 1 are be currently active (Norris and Johnson, Emperor chain has confirmed an increase in after the classification of Williams and 1969; Johnson, 1970). Some of the vol- age away from the active of others (1954). All samples we dated were canic islands in the Austral-Cook chain Kilauea, although the increase with dis- collected by A.E.J. Engel. Unfortunately, were described by Marshall half a century tance is only approximately linear the field notes describing the location of ago (1908, 1909, 1912, 1927, 1929, 1930) (McDougall, 1964; Jackson and others, these samples were destroyed, and although and by Chubb (1927a, 1927b). More re- 1972; Dalrymple and others, 1974; Clague the island from which each sample was col- cently, Wood (1967) and Wood and Hay and others, 1975). Relative motion between lected is known, the exact location and (1970) described the geology and petrology two melting spots can be detected by com- stratigraphic position for any of the sam- of the Cook Islands. paring the age-distance relation of two dif- ples on individual islands are not known The origin of linear volcanic chains is of ferent chains that have formed on the same with certainty. No ages were determined considerable current interest. Wilson lithospheric plate. The few available for Rapa, although suitable samples were (1963a, 1963b) recognized that several radiometric (Krummenacher and Noetzlin, available from the collections of L. J. chains in the Pacific trend generally west- 1966; Tarling, 1967) and paleontologic Chubb (Smith and Chubb, 1927) and (Marshall, 1927, 1930; Wood and Hay, 1970) ages for the Southern Cook and Aus- * Present address: (Jarrard) Department of Geology, tral Islands were only partly consistent 1 Copies of GSA supplementary material 75-29 may University of California, Santa Barbara, California be ordered from Documents Secretary, Geological Soci- 93106; (Clague) U.S. Geological Survey, Menlo Park, with the fixed-melting-spot hypothesis ety of America, 3300 Penrose Place, Boulder, Colorado California 94025. (Clague and Jarrard, 1973a). Further dat- 80301, USA.

Geological Society of America Bulletin, v. 86, p. 1463-1467, 1 fig., October 1975, Doc. no. 51018.

1463

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A.E.J. Engel, because Krummenacher and Group is overlain by rocks of the Muri date both the Te Manga Group and the Av- Noetzlin (1966) had obtained concordant Flows and the Te Kou Complex, both of the Caldera Complex. The ages reported ages of 5.1 ± 0.4, 5.0 ± 0.2, and 5.2 ± 1.7 extracaldera group. The phono- by Tarling not only disagree with our m.y. on samples from the island. Five sam- lite (PV575) is identical to one described by measurements but are stratigraphically in- ples collected from Atiu by Peter Wood Marshall (1908) from Muri Point and is consistent, and we conclude that they are were examined but were not dated because probably from the Muri Flows. The anor- probably incorrect. of alteration or high carbonate content. thoclase trachyte (PV504) might be from The six samples from Mangaia range in The main mass of Rurutu is alkalic either the Te Kou Complex, the Muri age from 16.6 to 18.9 m.y. The difference basalt. The island rises to an elevation of Flows, or the Maungatea Breccia, where between the oldest and the youngest calcu- about 400 m and is bounded by an elevated Wood and Hay (1970) described anortho- lated ages is statistically significant at the 95 limestone reef or "" approximately clase trachyte units. percent level of confidence, and the appar- 75 to 100 m above sea level and by a The flows exposed on Aitutaki are ent range of ages may represent a real dif- present-day fringing reef. The dated sam- primarily nepheline basalt and associated ference between the ages of the flows sam- ples from Rurutu are similar to Smith and mafic alkalic basalt, although on Rapota pled. Chubb's (1927) nepheline sample (an islet that is part of Aitutaki) they are Though the samples are unlocated, the 797 but are here called oligoclase basalt be- overlain by an agglomerate that contains relation between the dated samples and the cause nepheline was not found in thin sec- pebbles of trachyte and phonolite that were exposed lava flows of each volcano can be tion. Four samples collected in 1925 by possibly erupted during a late stage in the inferred. It appears that the oldest exposed Chubb were examined through the courtesy volcanic history of the island (Wood and parts of Mangaia and Rarotonga have been of the British Museum of Natural History, Hay, 1970). The stratigraphically oldest dated. For Aitutaki, the relation is less cer- but none was suitable for dating. flows exposed on the island are of nepheline tain. Wood and Hay (1970) observed that Mangaia Island is formed of low, deeply basalt; they are underlain in places by Maungapu (the inferred location of our eroded central hills of basalt surrounded by palagonitic ash and weathered basalt Aitutaki samples) is on the western edge of a makatea. Although the flows are poorly . The dated nephelinite samples are a craterlike feature that may have de- exposed, they appear to represent a single identical to those described by Wood and veloped during a rejuvenation of volcanic eruptive series, unbroken by any erosional Hay (1970) from near Maungapu (a prom- activity. Our samples, however, do not re- hiatus. The basalt samples that we dated inent peak on the north end of the island) semble the phonolite and trachyte described are similar to those described by Chubb and are probably from that area. by Wood and Hay (1970) as the products (1927a) and Wood and Hay (1970) and of the latest stage of volcanism. Rather, apparently were collected from the south or DISCUSSION they are similar in composition to the older west slopes of the central hills, the only lo- flows and more likely date the earlier of the cality where the rocks crop out. The ank- The K-Ar results (Table 1) indicate that two stages of volcanism exposed on the is- aramite samples that we dated are probably the samples from Aitutaki and Rurutu are land. The dated samples, however, are from Vau Roa Valley as they are identical young, having weighted mean ages of 0.75 nephelinitic, and the ages found may post- to those described by Chubb (1927a) and ± 0.03 and 1.05 ± 0.02 m.y., respectively. date the main constructional phase of Wood and Hay (1970) from that locality. (Weighting is by the inverse of the var- Aitutaki by several million years if Aitutaki The samples from Rarotonga represent a iance.) These data are in good agreement has had an eruption history similar to that broad range of rock types from several dis- with the ages of 0.7 ± 0.5 m.y. (Aitutaki) of Hawaiian volcanoes (discussed below). tinct formations. The ankaramite (PV573) and 0.5 ± 0.5 m.y. (Rurutu) obtained by The geology of Rurutu is not as well known and the analcite trachydiabase (PV587) are Krummenacher and Noetzlin (1966) on as that of the other islands; the descriptions similar to samples described by Wood and single samples and confirm that these is- by Smith and Chubb (1927) indicate that Hay (1970) from the Te Manga Group, lands are young. the main mass of Rurutu is alkalic basalt which forms the main mass of Rarotonga The ages obtained on samples from similar in composition to the samples and includes the oldest rocks exposed on Rarotonga indicate that it also is a young dated. the island. The Te Manga Group is primar- island and was formed in at least two dis- Because our samples are alkalic, not ily basalt, ankaramite, and limburgite. We tinct periods of activity. The two samples tholeiitic, there is some question about the infer that our samples are from the Te inferred to be from the Te Manga Group relation between the ages given in Table 1 Manga Group as similar rock types appar- have a weighted mean age of 1.79 ± 0.09 and the age of the main constructional stage ently do not crop out elsewhere on the m.y. Those thought to be from the of the volcanoes sampled. In , the island. post—caldera collapse phonolitic eruptions shields of the main constructional stage are After collapse of the caldera, emplace- are significantly younger; their ages range thought to have formed invariably by copi- ment of caldera-filling basalt flows, and a from 1.19 to 1.30 m.y. (Table 1). These ous and rapid eruptions of lava of the period of deep weathering and erosion, vol- ages differ from those measured by N. H. tholeiitic suite of Macdonald and Katsura canic activity on Rarotonga was resumed Gale and reported by Tarling (1967): 2.80 (1964). The average time required for con- with eruption of phonolitic flows. These ± 0.13 m.y. for a phonolite flow at Tuoru struction is probably of the order of 106 yr post—caldera collapse lava flows have been Quarry and 2.30 ± 0.24 m.y. for a sample or less (for example, Swanson, 1972; Jack- subdivided by Wood and Hay (1970), from Avatiu Valley. According to the son and others, 1972). The shield-building primarily on the basis of composition and geologic map of Wood and Hay (1970), the stage is followed by caldera collapse and by distribution, into two main groups — ex- flows accessible near Tarling's locations in eruption of lava flows of Macdonald and tracaldera and intracaldera — but at least Avatiu Valley are of the Avatiu Caldera Katsura's (1964) alkalic suite; these flows some rocks from both groups were proba- Complex and were erupted after the caldera form a thin mantle over the shields. Both of bly contemporaneous. The Maungatea collapse, which followed the extrusion of these events occur within a time span of Breccia fills conduits, vents, and craters as- the Te Manga Group; they should therefore approximately 105 yr. Lava flows of Mac- sociated with intracaldera phonolitic erup- be younger than 1.8 m.y. The phonolite in donald and Katsura's (1964) nephelinitic tions (Wood and Hay, 1970). The limbur- Tuoru Quarry belongs to the Raemaru suite commonly erupt from small satellite gitic basalt (PV574) is probably from the Flows, which are part of the later ex- vents after a period of quiescence and ero- Maungatea Breccia. Locally, the Te Manga tracaldera phonolitic eruptions and post- sion lasting about 1 to 2 m.y. However,

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RAIVAVAE

AGE,, A «-MANGAIA (Million RAPA Yrs) ATIU TUBUAlt i RAROTONGA MACDONALD- SEAMOUNT AITUTAKI RURUTU

I6°S 26°S

28°S

20°S

Figure 1. Bathymetry of the Cook-Austral chain and K-Ar ages as a function of distance from Shaded band represents the predicted age pattern based on the fixed-melting-spot hypothesis, the Macdonald Seamount. Filled circles, data from this study; open circles, data from Krummenacher Hawaiian pole of rotation (Clague and Jarrard, 1973b), and Hawaiian age data (McDougall, and Noetzlin (1966). Bars indicate analytical uncertainty at the 95 percent level of confidence. 1971; Jackson and others, 1972; Dalrymple and others, 1974; Clague and others, 1975). Arrows show minimum fossil ages from Marshall (1927, 1930) and Wood and Hay (1970). Bathymetry after Summerhayes (1967) and Mammerickx and others (1973).

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TABLE 1. K-Ar DATA ON VOLCANIC ROCKS FROM THE COOK AND AUSTRAL ISLANDS no more than three of the seven volcanoes dated can be accommodated by any Ar1*0 K 0f Weight rad 100 Arrâd Calculated age5 Sample no. Sample material* 2 reasonable linear or quasi-linear pattern. (wt. percent) (g) (mol/g) Ar (, 6 years total ° > Although the available age data for the

Rurutu Cook-Austral chain do not fit predictions

PV520 Oligoclase basalt 1.537 ± 0.006 (4) 7.858 0.2394 x IO"11 40.0 1.05 ± 0.03 based on the melting-spot hypothesis, there PV522 Oligoclase basalt 1.598 ± 0.017 (4) 9.176 0.2488 25.1 1.05 ± 0.03 are sufficient uncertainties in the interpreta- PV523 Oligoclase basalt 1.568 i 0.030 (6) 7.550 0.2516 33.9 1.09 ± 0.05 tion of the data to suggest that a firm con- PV524 Oligoclase basalt 1.721 ± 0.024 (4) 10.579 0.2582 43.7 1.02 ± 0.03 clusion regarding the validity of the Mangaia 1 hypothesis for the origin of this chain is 5.446 2.327 61.0 PV500 Ankaramite (1-2 tin) 0.849 ± 0.003 (4) 18.4 ± 0.4 4.726 2.324 31.1] probably not justified. Additional geologic, PV535 Ankaramite (1-2 mm) 0.292 ± 0.009 (8) 4.941 0.720 22.7 16.6 t 0.8 petrologic, and geochemical work is needed PV536 Basalt 0.705 ± 0.003 (4) 6.871 1.866 24.5 17.8 ± 0.6 to determine the eruption sequence and tim- PV537 Ankaramite (1-2 run) 0.714 i 0.011 (8) 4.968 2.009 48.1 18.9 1 0.7 ing of the shield volcanoes in the chain. If PV538 Ankaramite (1-2 mm) 0.267 ± 0.004 (7) 5.176 0.677 24.8 17.1 1 0.6 the shields are even partly tholeiitic, then 4.336 1.802 19.5 1 PV539 Basalt 0.706 ± 0.013 (8) 17.7 1 0.6 3.470 1.912 19.6 ? samples of tholeiite should be obtained Rarotonga from several of the volcanoes, perhaps by

PV504 Anorthoclase trachyte 5.16 ± 0.10 (8) 3.338 0.944 20.6 1.24 ± 0.08 dredging, and dated to determine the tem- PV573 Ankaramite (0.6-1.0 mn) 1.273 1 0.017 (8) 5.969 0.3300 13.1 1.75 ± 0.12 poral relation between the main shield- PV574 Limburgitic basalt 2.633 1 0.052 (8) 9.589 0.4650 35.2 1.19 ± 0.04 building stage and the exposed alkalic PV575 Nepheline phonolite 5.25 ± 0.08 (10) 5.254 0.935 5.7 1.21 ± 0.21 rocks. And additional K-Ar age measure- PV584 Amkaramitic analcite 0.924 ± 0.003 (4) 5.939 0.1769 19.5 1.30 ± 0.06 basalt ments on samples from and PV587 Analcite trachydiabase 1.683 ± 0.023 (8) 10.007 0.4556 13.8 1.83 ± 0.13 are needed to verify the large ap-

Ai tutaki parent range in ages of volcanic units from

PV532 nephelinite 1.052 t 0.014 (4) 11.463 0.1191 14.0 0.77 ± 0.05 these islands. Until such data are available, PV533 Olivine nephelinite 1.110 ± 0.008 (4) 9.244 0.1090 14.6 0.66 ± 0.06 confirmation or rejection of a melting-spot PV534 Olivine nephelinite 1.900 ± 0.014 (4) 9.629 0.2173 . 18.4 0.77 + 0.04 origin for the Cook-Austral chain will not be possible.

*Where a size fraction is not indicated, a single block was used. +Mean value and calculated standard deviation. The number of independent measurements given in parentheses. ACKNOWLEDGMENTS § 1 -1 10 _1 0 11 X£ = 0.585 x 10" yr , = 4.72 x 10' yr , K'' /Ktotal = 1.19 x lO" mol/mol. The errors are estimates of the standard deviation of precision (Cox and Dalrymple, 1967). We thank A.E.J. Engel, Peter Wood, and the British Museum of Natural History for Hawaiian volcanoes may become extinct at (early planeze stage); Rapa (early residual kindly making samples available to us for any stage of development within this se- mountain stage); Raivavae, Tubuai, and study. We also thank L. B. Schlocker, S. J. quence. For Hawaii, ages from lava flows of Rurutu (late residual mountain stage); Kover, A. H. Atkinson, and J. C. Von Essen the tholeiitic shield most nearly represent Mangaia (skeletal stage); and Aitutaki (late for laboratory assistance, and J. Nat- the time of inception of the volcanoes skeletal stage). Although the geomorphic land, E. D. Jackson, R. J. Fleck, and I. (Jackson and others, 1972). It is not ages in general increase to the northwest, McDougall for reviewing the manuscript. known, however, whether the volcanoes of the geomorphic age for Rarotonga, more This research was partly supported by the Cook-Austral chain follow the eruption than 2,200 km from Macdonald Seamount, National Science Foundation Grant pattern and timing of Hawaiian volcanoes. is inconsistent with the pattern. The GA—36589. The absence of tholeiite in these islands geomorphic sequence and K-Ar data also seem to be inconsistent; the samples from suggests either that the alkalic cap typically REFERENCES CITED is much thicker on Cook-Austral than on Aitutaki, for example, are younger than those from Rarotonga. Hawaiian volcanoes or that lava flows of Betz, F., Jr., and Hess, H. H., 1942, The floor of the tholeiitic suite are completely absent. It is clear from the age plot (Fig. 1) that the North Pacific Ocean: Geog. Rev., v. 32, It has long been asserted, on geomorphic the data for the Cook-Austral chain are not p. 99-116. evidence, that the Cook-Austral chain in- even broadly linear and are inconsistent Chubb, L. J., 1927a, Mangaia and Rurutu: A creases in age to the northwest (Chubb, with any simple hypothesis that all the vol- comparison between two Pacific islands: 1957). The prediction of relative ages on canic rocks of the chain formed by plate Geol. Mag., v. 64, p. 518-522. geomorphic grounds requires the assump- motion over a single fixed melting spot. 1927b, The geology of the Austral or Only two volcanoes, Rapa and Mangaia, Tubuaii Islands: Geol. Soc. London Quart. tion that rates of erosion have been reason- Jour., v. 83, p. 291-316. ably uniform. Many factors such as varia- are reasonably close to the pattern (shaded 1957, The pattern of some Pacific island tions in rock type, rainfall, and reef de- band in Fig. 1) predicted using the age data chains: Geol. Mag., v. 94, p. 221-228. velopment challenge this assumption and from the Hawaiian chain (McDougall, Clague, D. A., and Jarrard, R. D., 1973a, Ter- suggest that geomorphic ages be used with 1971; Jackson and others, 1972; Dalrym- tiary Pacific plate motion deduced from the caution. The use of geomorphic criteria to ple and others, 1974; Clague and others, Hawaiian-Emperor chain: Geol. Soc. establish relative ages for islands in the 1975) and the Hawaiian pole of rotation America Bull., v. 84, p. 1135-1154. Cook-Austral chain may be further compli- that was calculated by Clague and Jarrard 1973b, Hot spots and Pacific plate motion cated by a complex history of emergence (1973b). The volcanic rocks of Aitutaki, [abs.]: EOS (Am. Geophys. Union Trans.), v. 54, p. 238. and submergence, for which there is abun- Rarotonga, and Rurutu are much younger Clague, D. A., Dalrymple, G. B., and Moberly, than predicted, whereas some samples from dant evidence (Wood and Hay, 1970). R., 1975, Petrography and K-Ar ages of Using the erosion stages of Cotton (1944) Tubuai and Raivavae appear to be dredged volcanic rocks from the western and of Kear (1957), the dated islands can be significantly older than predicted. Even if Hawaiian Ridge and southern Emperor ranked in order of increasing apparent the Cook-Austral melting spot is allowed to Seamount chain: Geol. Soc. America Bull., geomorphic age as follows: Rarotonga move relative to the Hawaiian melting spot, v. 86, p. 991-998.

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